The problem of routing conductive traces in densely-packed PWBs has been at the forefront of electronics circuit design and manufacture for decades. On multi-layer PWBs, most of the traces lie on between the internal conductive layers of the PWB. In order to effect connections between the conductive layers, conductive vias are required. Vias have traditionally been formed as through-hole vias: by drilling holes through the entire PWB after the multiple layers have been assembled and then plating the holes with a conductive material, such as copper, so as to connect traces or solder pads on one layer to those on another one or more layers. During the PWB design process, as more and more vias are placed on the PWB, the task of routing the traces on the various conductive layers becomes more and more difficult, because the vias represent blockages for traces that do not connect to them. The recent introduction of ball-grid array (BGA) devices, whose "pins" (solder balls) are arranged in a tightly-packed two-dimensional array, exacerbate this routing problem, because the density of the "pin" array creates a virtual "bed-of-nails" via structure within the PWB. Such a large number of obstructions can make the routing problem virtually impossible to solve.
Recently, a significant improvement in via technology has been introduced, called micro-via. Micro-vias are very small blind vias. A blind via is one that extends from a surface of the completed PWB to one of the interior conductive layers of the PWB. The holes for micro-vias are typically laser-drilled for precision and for lower cost. Such micro-vias do not create blockages on PWB layers to/through which they do not extend. Nevertheless, they create as much of an obstruction in the layers through which they do extend as the through-hole vias.
Blind vias can be created by controlling drill depth or by drilling a laminated sub-component of a multi-layer PWB prior to final lamination. The latter adds cost to the assembly process and does not afford the opportunity to connect buried vias to blind vias. The former technique has been viable with mechanical drills, but only for relatively large-dimensioned PWB designs, due to loose mechanical tolerances.
Vias, whether through-hole or blind, generally attach to a layer at a pad, as do the contacts ("pins") of components at the outer surface layers of the PWB. However, if the contact of a component is attached to the same pad as a via, there is the danger--which increases with the diameter of the via--that the via will wick the solder away from the contact and thereby cause an unreliable connection. For this reason, separate pads have generally been used on the outer surface layers of the PWB for attaching vias and for attaching component contacts. But, like the vias, the additional pads create routing blockages on the surface layers of the PWB and they limit the number of connections that can be made to, and hence the number of components that can be mounted on, the PWB. It is therefore desirable to put vias in the component pads, if the solder-wicking problem can be overcome, so as to allow additional trace routing on the outer surface layers.